1. Field of the Invention
The present invention relates to a frequency converting circuit comprising a PLL (Phase-Locked Loop) circuit, and, in particular, relates to a damping circuit for locally oscillated electric signals.
This application is based on Patent Application No. Hei 11-160256 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
It is general practice to convert input signals mixed with signals which are generated by a locally oscillating circuit (hereinafter, the abbreviation "OSC" indicates oscillating) into output signals with a predetermined frequency. A frequency converting circuit ideally outputs signals with only output frequency, however in practice, output signals contain leaked components originating from the input signals or the OSC signals. It is required that the amount of signal leakage be reduced to as low as possible since the output signals can be undesirably affected if the leaked components from the input signals or the OSC signals are excessive. Furthermore, in the case of a frequency converting circuit comprising a known double-balanced mixing circuit, the leaked components from the OSC signals are greater than the ones from the input signals, and the leaked components must be reduced.
In order to meet the above requirements, a circuit, shown in FIG. 5, for example, has been proposed. In this circuit, a signal being input from a terminal 51 and the OSC signal being output from the OSC circuit 54, which is controlled by a PLL circuit 57 fed by a buffer amplifier 56, are mixed in a mixer 55, then a differential frequency is derived. The derived signals are amplified by buffer amplifier 58 passed from terminal 52 through a capacitor 59, a filter 60 (such as a band-pass filter or a low-pass filter) and a further capacitor 61 in order to eliminate undesired leak signals, so that only necessary output signals can be obtained at terminal 53.